Hemoglobin Ao (HbAo), a potential blood substitute, potentiated submaximal agonist-induced platelet aggregation without increasing serotonin release or cytosolic calcium elevation or fibrinogen binding. The predominant mechanism for this potentiation of platelet aggregation was the cyclooxygenase activity of hemoglobin, which can catalyze metabolism of arachidonic acid into prostaglandin (PG)F2a. Levels of PGF2a release by activated platelets were found to increase significantly in the presence of hemoglobin. Addition of this prostaglandin potentiated ADP-induced platelet aggregation but not secretion, thus mimicking the effect of HbAo. These observations suggest that prostaglandins play a central role in the mechanism of hemoglobin potentiation of platelet aggregation. Induction of aggregation without additional platelet activation led us to study different conformations of GPIIb/IIIa which could bind fibrinogen. Ligand induced binding sites (LIBS) are neoantigenic regions of GPIIb-IIIa that are exposed upon interaction of the receptor with the ligand fibrinogen The GPIIb-IIIa complex-activating antibody, anti-LIBS D3, induces fibrinogen and von Willebrand factor binding to GPIIb-IIIa on intact platelets but does not increase the binding of another ligand, vitronectin. Thus, the region of GPIIIa recognized by D3 may be an important regulatory domain in ligand-receptor interactions that directly mediate platelet aggregation. Peroxynitrite (ONOO), a free radical produced by the interaction of NO with hydrogen peroxide, induces nitration of tyrosine residues and inhibits tyrosine phosphorylation in cell free systems. We investigated the effect of ONOO on protein tyrosine nitration and phosphorylation in resting or thrombin-activated platelets. ONOO rapidly induced tyrosine nitration of proteins in gel-filtered platelets which persisted up to 4.5 hours and also rapidly increased tyrosine phosphorylation of a separate set of proteins which decreased by 5 minutes. Thrombin induced the same pattern of tyrosine phosphorylation in platelets, and this was not altered by pretreatment of platelets with ONOO. Thus, tyrosine nitration does not alter thrombin-induced platelet tyrosine phosphorylation and inhibitory effects of ONOO on platelets can be independent of tyrosine nitration. The administration of measles vaccine is associated with low level incidence of thrombocytopenia. An antiplatelet antibody was found in the vaccine recipients who developed thrombocytopenia and this antibody recogized purified GP IIIa. The measles viral proteins appear to have immunogenic homology to platelet GPIIIa.